The present invention relates to a radio frequency power amplifier that operates efficiently when used for amplifying signals of varying amplitude, and that may be switched to a more efficient mode of operation for amplifying a signal of constant amplitude. The invention further relates to a radio frequency power amplifier that provides higher efficiency when operation at alternate output levels is needed, with either constant or varying amplitude signals.
Radio transmission of signals and information takes place by impressing the signals on a radio frequency wave known as the "carrier", the process of impressing the information being known as "modulation". The most often used methods of modulation are the two special classes known respectively as Amplitude Modulation (AM) and Frequency Modulation (FM).
In AM, only the amplitude or strength of the radio frequency (RF) carrier is varied by the information-bearing signals, the frequency of the RF carrier being constant. In FM, only frequency of the RF carrier is varied, the amplitude being constant.
Frequency modulation is one of a wider class of constant-amplitude modulations in which the phase angle variation of the carrier carries the information. Constant amplitude modulations are the easiest for which to construct an efficient transmitter, as their design can be optimized to give maximum efficiency at the one and only output power level used.
Amplitude modulation is more difficult to generate efficiently, and usually requires a different design of transmitter.
A more general class of modulations, called "complex modulation", permits both the amplitude and phase angle of the carrier wave to be varied. This class requires use of a "linear" type of transmitter power amplifier in order to accurately reproduce the modulated amplitude of the input waveform, and consequently is the most difficult in which to achieve high efficiency while simultaneously maintaining linear operation and avoidance of intermodulation distortion. The most common modulation falling in this class is SSB (Single SideBand) modulation.
A number of different RF power amplification techniques are known in the art. These will now be described.
1. High Level Modulation
The most efficient way to generate a high-power amplitude-modulated signal is the so-called high-level modulation method, where the low-frequency modulating signal itself is strongly amplified and used as a varying power supply for the high power RF amplifier. In this system, the RF power amplifier can operate at maximum efficiency in the class-C regime as in the case of FM, as the output power variations are produced by varying the input power supply proportional to the instantaneous power level desired.
A high-level modulator must generate up to half the transmitter output carrier power and is therefore a large, heavy and costly element. This makes this type of amplifier unsuitable for use in applications where size, weight or cost considerations are more important than efficiency of power generation. In such cases, low-level modulation schemes are sometimes used, instead. A typical low level modulation scheme generates the amplitude modulated RF carrier first at a low power level where the efficiency of the method is immaterial, and then amplifies it to a high power level using a linear power amplifier, similar to that used for complex modulations such as SSB.
2. Linear Power Amplifiers
The classic forms of linear power amplifiers operate in the regimes known as class-A or class-B.
2.1 Class-A Power Amplifier Efficiency
A class-A amplifier device is biased to continuously consume, even when not required to produce any output power, a mean current equal to half the peak current that it draws when generating maximum output power. When driven with a maximum amplitude AC input signal, such as a sinusoidal radio carrier frequency signal, the class-A amplifier current swings between zero and twice the mean current, but the average over a cycle of the input signal remains constant. The signal power output under these circumstances is a maximum, and the efficiency is a maximum of 50%.
When driven by an AC input signal of less than the maximum amplitude, the class-A current swings between corresponding fractions of the mean current, but the average current is still constant. Taking, for example, the case of an AC input signal of half the maximum amplitude, the class-A current swings between 0.5 and 1.5 times the mean current, but the average current is still equal to the mean current. The signal power under these conditions is only one quarter the maximum power output, but the current consumption from the supply remains the same high value as for maximum power output. The efficiency at lower outputs is thus reduced, being only 12.5% at half amplitude, quarter power output conditions.
An RF signal that is varied in amplitude symmetrically about a mean amplitude by an amplitude modulating signal, the maximum amplitude variation being from zero through the mean to twice the mean (i.e., amplitude varying from mean-mean to mean+mean), must be adjusted so that the peak amplitude of twice the mean corresponds to less or equal to the maximum power condition of the class-A amplifier mentioned above, in order to avoid distortion. It is thus only at that momentary peak amplitude that the class-A power amplifier delivers its maximum efficiency of 50%, and the average efficiency averaged over a modulating cycle is reduced. Under conditions where the modulating signal is quiescent and the amplifier is driven to an RF mean output level equal to half the peak RF output level (the unmodulated carrier condition of an AM transmitter), the efficiency for generating the quiescent carrier is only 12.5%.
2.2 Class-B Power Amplifier Efficiency
The low efficiency of the class-A amplifier in the mean is due to its not reducing its power consumption when the output power requirement is at a low point. This disadvantage is partially overcome by the class-B amplifier.
The classic class-B amplifier consists of two identical amplifier devices arranged in a push-pull configuration and biased just at the point where they are not consuming any current from the supply when no RF input drive signal is present. If the RF input drive signal swings in the positive direction, one of the devices will begin to take current proportionally, while the other takes over on the negative half cycles.
Each device in the class-B amplifier thus takes a half-sine shaped burst of current with a mean value of 1/.pi. times the peak. The total current taken by both devices from the supply is then 2/.pi. times the peak current. The total current increases with RF drive level, thereby increasing the signal delivered to the load (the antenna) proportionally until the output voltage swing developed across the load impedance is almost equal to the available power supply voltage. A further increase in RF drive cannot increase the voltage developed across the load beyond this point, and the amplifier is said to have saturated, or to be "clipping". For linear modulation applications, the amplifier must not be driven into this region, or else distortion of the modulation will occur.
The maximum peak current drawn by each of the push-pull devices just before clipping is equal to the power supply voltage V.sub.o divided by the load impedance R.sub.L. EQU Imax=V.sub.o /R.sub.L
The mean current being 2/.pi. times the peak, we obtain the mean input power from the supply to be: EQU V.sub.o .multidot.I.sub.max .multidot.2/.pi.=(2/.pi.)V.sub.o.sup.2 /R.sub.L
while the AC power developed in the load R.sub.L is 0.5V.sub.o.sup.2 /R.sub.L. Taking the ratio of output AC power to power consumption gives the efficiency .pi./4 or 78.5%.
The efficiency at lower than peak amplitude is calculated by observing that the power consumption reduces in proportion to the output voltage swing while the power output reduces in proportion to the square of the output voltage swing. The efficiency thus reduces linearly with output amplitude, so that at a mean amplitude equal to half the peak, the efficiency is equal to half the peak efficiency, that is 0.5.pi./4 or 39%.
This is a considerable improvement upon the class-A amplifier's 12.5% efficiency at half its peak amplitude level, and occurs because the class-B amplifier reduces its current drawn in proportion to the demand. It is however still a low efficiency compared to the 78.5% of the class-B power amplifier at full output. The theoretical efficiency of the non-linear class-C amplifier is even better, approaching 100%, so there is a large penalty in efficiency for low-level modulated AM transmitters compared to FM transmitters.
3. Load Impedance Switching
Where a transmitter must sometimes work with AM signals and sometimes with FM signals, the reduced efficiency of the AM design usually still pertains in the FM mode. This can be problematic in applications such as portable radiotelephone transmitters that, because of small size requirements and reliance on battery power supplies, require that a single power amplifier operate efficiently in alternative AM and FM modes of operation, and have the same mean power output level in both cases.
If a class-B amplifier is to be used for a 1 watt carrier power AM transmitter, it must be capable of generating 4 watts on modulation peaks at which the amplitude is doubled. If such an amplifier were then used for a 1 watt FM transmitter, its efficiency would only be 39% instead of the 78.5% theoretically possible. In battery operated equipment such as handheld radios, the choices of either running the power amplifier at 1 watt and half efficiency or 4 watts with full efficiency both have a negative impact on battery life.
Several techniques are known which address this problem.
U.S. Pat. No. 5,060,294 to Schwent, which is incorporated herein by reference, describes an amplifier in which the load impedance is switched between a first value which causes the amplifier to saturate and a second value which prevents the amplifier from saturating. This permits the amplifier to be used with either constant or varying amplitude signals. However, Schwent is silent on achieving alternative power output levels at improved efficiencies.
U.S. patent application Ser. No. 08/061,345 to Dent, filed on May 17, 1993, which is also incorporated herein by reference, discloses an amplifier which employs load impedance switching to achieve optimum efficiencies at alternative output power levels, the alternative output power levels corresponding to amplifier operation in either a linear (e.g., class-B) mode in both cases for use with varying-amplitude signals or a saturated (e.g., class-C) mode in both cases for use with constant amplitude signals. The use of impedance switching, however, has a number of drawbacks, including the losses associated with the switches and the abrupt transition from one mode to another. Furthermore, the efficiency of this type of amplifier is low when operating in a linear mode.
It is therefore desirable to provide an RF power amplifier that can operate in a linear (e.g., class-B) mode at one power level and in a saturated (e.g., class-C) mode at an alternative output power level without the use of load impedance switching or the use of alternative power supply voltages.